Stress orientated Hydrogen Induced cracking (SOHIC) is a manifestation of sulfide stress cracking (SSC) in C-Mn steels by a mechanism involving the linking of planar cracks normal to the primary propagation direction of the crack. It has been known for many years, and the solution has been considered to be the use of hydrogen induced cracking (HIC) resistant steels. Work by the Materials Properties Council has, however indicated that HIC resistant steels may be more susceptible to SOHIC. This raises questions about both the most appropriate steels, and test methods for evaluating them
Published literature has been surveyed, and information pertaining to the mechanisms of SOHIC, with particular reference to the effects of material, fabrication, and service conditions on the risk of cracking, has been extracted. A critical review of this information, along with published information on test methods and results, and information on industrial experience including service failures due to SOHIC, has allowed conclusions to be drawn and recommendations to be made on materials selection, fabrication and testing for resistance to SOHIC.
Stress Orientated Hydrogen Induced Cracking (SOHIC) is a manifestation of sulfide stress cracking (SSC) in C-Mn steels, by a mechanism involving the linking of planar cracks normal to the primary propagation direction of the crack. It has been known for many years, and was indeed designated Type I cracking by workers from Sumitomo Metal Industries (1), with the conventional, hardness dependent, SSC being referred to as Type II (Fig.1). Similarities with hydrogen induced cracking (HIC, also termed Hydrogen Induced Cracking or Stepwise Cracking) were noted, and the solution was considered to be use of HIC resistant steels (1-3). Even at that time, however, there was some uncertainty over the strength of the correlations between HIC resistance, and resistance to SSC (4) . More recently, work funded by the Materials Properties Council in USA has indicated that HIC resistant steel may indeed be more susceptible to SOHIC than conventional steels (5) . Thus, purchasers are left with some uncertainty over the appropriate steel quality required to provide resistance to SOHIC. Before exploring published evidence on factors affecting SOHIC, however, it is necessary to define the phenomenon, so that earlier papers, which have used different terminology, can be assessed correctly.
One factor which has been identified from early on as affecting the risk of SOHIC, is welding (6) . Specific attention has therefore been given to papers which have explored the effects of welding on microstructure and properties which could have had a bearing on SOHIC.
Much of the current concern about SOHIC arises from industrial failures, believed to have been by this mechanism. An important part of this review, therefore, has been to obtain as much detail as possible about such failures, with a view to identifying common factors.
In the following Sections, therefore, after considering the definition of SOHIC and clarifying steel terminology, the fundamental mechanism of SOHIC and microstructural effects are explored before discussing the effects of welding and post weld heat treatment.